1. Field of the Invention
This invention relates to copolymers of maleic anhydride and a C1-C4 alkyl vinyl ether, and, more particularly, to a continuous, solvent-free process for making solvent-free, fine white powders of such copolymers having advantageous properties useful for oral care compositions.
2. Description of the Prior Art
A batch, solvent-free process (i.e. no solvent other than reactants themselves) for making fine powders of high molecular alternating copolymers of maleic anhydride (MAN) and a C1-C4 alkyl vinyl ether (AVE) having a molecular structure of (A-B)n where A=MAN, B=AVE and n is an integer which is indicative of its molecular weight, is described in U.S. Pat. Nos. 5,959,053 and 5,939,506. In such process, excess AVE monomer is present over a 1:1 molar ratio of monomers, to function both as reactant and solvent in the system. No other solvent is present in the reactor. Accordingly, after stripping the excess AVE from the reaction product, a solvent-free copolymer is obtained. However, a batch process requires an extended amount of time to effect the reaction, usually necessitating about 9-13 hours to complete each batch.
Accordingly, it is an object of this invention to provide a continuous, solvent-free process for making solvent-free, fine white powders of copolymers of MAN and AVE having advantageous properties useful for oral care compositions.
What is described herein is a continuous, solvent-free process for obtaining solvent-free, fine white powders of high molecular weight alternating copolymers of maleic anhydride and a C1-C4 alkyl vinyl ether, without odor or taste, having the molecular structure (A-B)n, where A is maleic anhydride, B is an alkyl vinyl ether, and n is an integer indicative of the molecular weight of the copolymer. The process herein comprises feeding continuously alkyl vinyl ether as monomer reactant and solvent in excess of the desired 1:1 mole ratio of A:B in the copolymer, molten maleic anhydride and a free radical initiator, at predetermined feeding rates, into a reactor heated at a reaction temperature of about 50xc2x0 to 100xc2x0 C., charging the reactor and withdrawing reaction product, residual alkyl vinyl ether and initiator from the reactor at the same rate as the feed rate of reactants into the reactor simultaneously thereby maintaining a constant volume in the reactor, stripping residual alkyl vinyl ether from the reaction product, drying the copolymer obtained of any remaining traces of alkyl vinyl ether, and recycling recovered alkyl vinyl ether back into reactor.
In the preferred embodiments of the continuous solvent-free process of the invention, the alkyl vinyl ether is methyl vinyl ether, the mole ratio of methyl vinyl ether to molten maleic anhydride is about 25:1 to 5:1, most preferably 12:1 to 7:1. The initiator concentration is 0.01 to 0.2% based on the weight of copolymer, and the % copolymer solids removed from the polymerization reactor is about 10 to 40%.